1. Field of the Invention
This invention relates generally to the field of semiconductor circuit design, and more particularly to the design of a programmable metal or VIA element in an integrated circuit layout.
2. Description of the Related Art
Integrated circuits (ICs, also commonly referred to chips) are well known components that can be found in nearly all electronic equipment in use today. The integration of large numbers of small transistors into a single chip represents a sizeable improvement over the assembly of circuits comprising discrete electronic components. Two of the main advantages of ICs over discrete circuits are cost and performance. Cost is reduced because ICs are printed as a unit using photolithography. Consequently, producing a circuit in the form of an IC requires much less material than producing a corresponding circuit built from discrete components. Performance of ICs is high, since the components typically switch quickly and consume little power compared to their discrete counterparts.
The concentration of transistors on ICs has risen over the years, with the first integrated circuits containing transistors numbering in the tens (Small-Scale Integration, or SSI). The next developmental step led to ICs containing hundreds of transistors on each chip (Medium-Scale Integration, or MSI), with further development leading to tens of thousands of transistors per chip (Large-Scale Integration, or LSI), leading to present day ICs comprising anywhere from hundreds of thousands of transistors to a billion transistors (Very Large-Scale Integration, or VLSI). Manufacturing has been tending towards smaller and smaller channel lengths and cleaner fabrication facilities, enabling the production of ICs with more transistors at adequate yields. Design tools have also been steadily improving, successfully reducing design cycles and time to market of various designs, such as microprocessors, graphics processing chips and memories. The most commonly used process is based on the more energy efficient CMOS (Complementary Metal-Oxide Semiconductor), which has replaced NMOS-only and PMOS-only designs to avoid prohibitive increases in power consumption.
In many instances, all the components required for a given digital or computer system may be included on a single IC, which is commonly referred to as System-on-a-Chip (SOC). ICs are typically composed of specific overlapping layers, each defined by photolithography, and normally associated with different colors during the design process. The layers currently defined include the diffusion layer, which specifies where various dopants might be diffused into the substrate, the implant layer, which specifies where additional ions are implanted, the polysilicon and metal layers that constitute the conducting layers, and the VIA (or contact) layer, which defines the connections between the conducting layers. All components are generally constructed from a specific combination of these layers. In a typical CMOS process, a transistor is formed wherever the gate layer (polysilicon or metal) crosses a diffusion layer.
The various layers on an IC are obtained through a process that is akin to a photographic process, but using higher frequency light, typically ultraviolet to create the patterns for each layer. The patterns are ultimately provided in form of a template referred to as a mask, which determines how the various components are configured and interconnected on the IC. Each IC is tested before packaging, typically using automated test equipment, during what is commonly known as wafer testing, or wafer probing. Following wafer probing, each wafer is typically cut into blocks, each block referred to as a die. Each good die is then connected into a package. Since each feature is very small, electron microscopes have become essential tools for process engineers to debug a fabrication process and/or fabricated prototypes. Various methods have also been developed to modify ICs after initial manufacture, without necessarily having to implement each experimental change in a new mask, or without having to make major modifications to obtain a new mask.
One solution for modifying an IC after fabrication is the use of focused ion beam machines (FIB, for short), which employ a technique for site-specific analysis, deposition, and removal of portions of various layers on a manufactured IC. Because of its sputtering capability, the FIB is oftentimes used as a micro-machining tool, to modify or otherwise machine materials at the micro- and nano-scale. FIB machines are often used in the semiconductor industry to patch or modify portions of an existing IC. For example, the gallium beam generated in the FIB machine is used to cut unwanted electrical connections, or to deposit conductive material in order to make a connection in an IC. Due to ever shrinking design cycles and time-to-market requirements, it has become important to have the capability to easily and quickly program various portions of an IC, such as a revision id register, for example, when making modifications to an IC after initial manufacture. However, the problem of programming on any single metal layer or any single VIA layer an infinite number of times, without having to make considerable layout modifications (or modifications to the production mask) has not yet been properly addressed.
Many other problems and disadvantages of the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.